1. Field of the Invention
The present invention relates to a battery assembly.
2. Description of the Related Art
Secondary batteries having a large capacity are employed in various applications where high energy and high power are required, such as vehicles, stationary batteries, or the like. In recent years, those batteries have been used as a main power source for driving electric motors in a pure electric vehicle (PEV) and a hybrid electric vehicle (HEV), which includes an engine and electric motor. With the growing need for protection of the global environment and for the effective utilization of energy resources, secondary batteries for PEVs/HEVs are expected to be in great demand in the future.
A battery assembly is used as a large-capacity secondary battery for a PEV/HEV driving power source. The battery assembly is built by connecting a plurality of battery modules, each of which includes two or more cells. The battery module is provided with a container that houses a plurality of electrode groups corresponding to the number of the cells and an electrolyte. The opening of the container is closed with a container cover. The container cover has a safety valve for releasing the gas in the container when the internal pressure of the container raises excessively.
In particular, a battery assembly used in HEVs is charged/discharged repeatedly in the range of 30 to 75% SOC (state of charge) during the normal driving of a vehicle. When the variation in the SOC between battery modules occurs with increasing running time of a vehicle, charge equalization is necessary at regular intervals to reduce the variation. The charge equalization usually is carried out so that the SOC of the battery assembly ranges from 100 to 120%. However, even in the charge equalization, the internal pressure of the battery modules to be charged may be increased. As shown in FIG. 5, a conventional battery assembly is provided with a safety valve whose working pressure is larger than the maximum internal pressure of the most highly charged battery module during the charge equalization, i.e., about 0.95 to 1.05 MPa.
As shown in FIG. 5, the maximum internal pressure of the most highly charged battery module during the charge equalization may be larger than the pressure that the container resists under elevated temperature conditions. However, a conventional battery assembly does not have a means for preventing a rise in the internal pressure during the charge equalization under elevated temperature conditions. Thus, it is possible that the container is deformed.
To solve the above problem, a method in which the pressure resistance of the individual containers is enhanced by increasing the thickness thereof is possible. However, a larger container thickness reduces the heat dissipation in battery modules, so that the battery temperature is apt to be raised, resulting in a vicious circle. In addition, the battery size, the cost, or the like is increased as well.